The present invention relates to a method for the preparation of anisotropic permanent magnet by a powder metallurgical technique. More particularly, the present invention relates to a method for the preparation of an anisotropic permanent magnet including a step of shaping a magnetic powder into a form by compression in a magnetic field to orient the magnetic particles, in which the magnetic particles can be oriented more completely within a greatly decreased time than in the conventional method.
It is a conventional process in the method for the preparation of an anisotropic permanent magnet by a powder metallurgical technique that particles of a magnetic alloy powder are oriented relative to the easy magnetization axis of the crystallites in a magnetic field using an electromagnet and then shaped by compression in a molding die followed by sintering. Such a process of molding is referred to as a process of field pressing hereinbelow. The magnetic field in conventional field pressing processes is of course static and usually has a strength of a few kOe to 10 kOe in most cases. A problem in this case is the incompleteness of the particle orientation for several reasons, and orientation of particles in the powder compact cannot be so complete as in a single crystal. Several of the reasons therefor include the difficulty in obtaining a sufficiently strong magnetic field, imperfect parallelism of the magnetic field, uneven compressive force on the powder compact in the compression shaping, non-uniformity in the impregnation of the molding die with the magnetic powder and so on.
The field pressing processes can be classified into two classes relative to the directions of the magnetic field and the compressive force. Namely, the direction of the magnetic field can be perpendicular to or parallel with the direction of the compressive force. It is usually understood that the latter method of powder compression in a direction parallel to the direction of the magnetic field, which is referred to as the parallel-field pressing hereinbelow, is less preferable, because of the disturbed orientation of the particles, than the former method, referred to as the transverse-field pressing hereinbelow, in which the oriented magnetic particles are compressed perpendicularly to the direction of the magnetic field. For example, a rare earth-cobalt magnet prepared by the parallel-field pressing has a saturation magnetization 4.pi.M.sub.z as a measure of the particle orientation lower by almost 10% than the magnet of the same rare earth-cobalt alloy prepared by the transverse-field pressing.
Although the degree of particle orientation can be improved by increasing the uniformity of the compressive force, use of a press with static hydraulic pressure such as a so-called rubber press is not always practical due to the unduly long time taken for a shot of molding and the difficulty in the design of the press by combining the press with an electromagnet built in. The degree of particle orientation can of course be improved by increasing the strength of the static magnetic field in the field pressing to several tens of kOe or higher. While a conventional electromagnet can produce a static magnetic field of up to 10 kOe in a space of a 10 to 100 mm gap, it is an extremely difficult matter to obtain a still stronger static magnetic field without using a superconducting magnet or a solenoid coil of normal conduction, but they are far from practical as an industrial means due to the expense of the apparatus, high costs for maintenance and low operability. Accordingly, it has been eagerly desired to develop a method for obtaining a high degree of orientation of magnetic particles, without the problems associated with the powder metallurgical method, for the preparation of an anisotropic permanent magnet.
Separately from the above described problems, to the prior art also includes preparation of an anisotropic permanent magnet in which the magnetic particles are oriented radially or in a plural number of radial directions. In the field pressing using a hydraulic press, for example, the molding die filled with the magnetic particles is surrounded by an electromagnet having a plurality of poles so as to realize the above mentioned particle orientation in a plurality of radial directions. Alternatively, magnet poles of the same polarity are oppositely disposed so as to obtain the radial orientation of the magnetic particles by utilizing the repulsion of the magnetic fields. These methods have several disadvantages such that the electromagnet is necessarily very large with low versatility in respect of the number of poles, and that such an electromagnet usually cannot produce a sufficiently strong magnetic field essential for obtaining a high degree of particle orientation.
In the injection molding of a plastic magnet, on the other hand, the orientation of the magnetic particles can be considerably high even without applying a particularly strong magnetic field because a mixture of magnetic particles and a molten binder resin is injected under a shearing force into a molding die. Of course, the performance of a plastic magnet inherently can never be so high as that of a sintered permanent magnet prepared by the powder metallurgical techniques because a plastic magnet comprises the non-magnetic binder resin in a considerably high volume fraction. For example, the maximum energy product (BH).sub.max of a plastic magnet composed of a magnetic powder and a binder resin in a volume ratio of 70:30 is sometimes only about 50% or even smaller compared to that of the sintered magnet prepared of the same magnetic powder. In the conventional design of magnets, accordingly, isotropic magnets are used in place of the above described radially oriented anisotropic permanent magnets notwithstanding the much lower values of the magnetic parameters than the anisotropic magnets, including about a half of the residual magnetization B.sub.r and about one fourth of the maximum energy product (BH).sub.max.